Various types and configurations of seal arrangements, bushings, packing boxes, stuffing boxes and the like are known for providing a seal around the rotating shaft that drives a ship's propeller, where the shaft penetrates through the stern of the ship from the interior to the exterior of the ship's hull. The seal arrangement must effectively prevent water from the exterior environment, i.e. from the exterior of the hull below the waterline, leaking into the interior of the hull where the shaft penetrates through the hull, while still allowing the shaft to rotate. Preferably, the seal arrangement must also dissipate the frictional heat that arises due to rubbing friction of the seal elements on the rotating shaft or the shaft bushing, and further should provide a long operating life and allow easy maintenance.
One known type of seal arrangement uses plural seal rings spaced axially from one another along the propeller shaft or shaft bushing, whereby the seal rings are held by a stationary support system such as a housing and/or stern tube. The seal rings may be embodied as lip seals and/or slide ring seals that make sliding contact with the shaft bushing provided around the propeller shaft. Even though various different configurations or embodiments of such seal arrangements are known, it has been found in practice that further improvements would be desirable. For example, it has been found that the seal rings, which are typically made of an elastomeric synthetic plastic, are negatively influenced by the high temperatures arising in the seal rings and in the surrounding media during operation, i.e. rotation, of the shaft, due to the frictional heating caused by the frictional sliding contact of the seal rings on the shaft or shaft bushing and due to contact with the hot lubricating oil.
Namely, the elastomeric plastic seal rings suffer a degradation and breakdown of the plastic material over time due to the arising high temperatures, and therefore suffer more rapid wear and deformation. This results in a disruption or deformation of the proper geometry of the seal gap between each seal ring and the shaft bushing, and thus permits leakage of the liquid media (e.g. seawater and/or lubricating oil) through the seal arrangement.
More particularly, on one side a portion of the seal arrangement is in contact with or exposed to the exterior seawater, which is generally at a relatively cool temperature, and on the other side another portion of the seal arrangement is in contact with or exposed to the lubricant in the stern tube, which lubricant is typically at a relatively warm or hot temperature. Namely, the lubricant, typically a lubricating oil, becomes heated due to frictional heating of the ball bearings, roller bearings or slide bearings that support the propeller shaft as the shaft rotates during operation. An additional significant heating contribution arises locally in the seal gap of the seal arrangement due to the frictional sliding contact of each seal ring on the shaft bushing. As a result, in view of the relatively low thermal conductivity of the elastomeric plastic material of the seal ring, this can give rise to a strong temperature increase especially directly at the seal lip of each seal ring. Due to the existence of different heated areas (e.g. friction at the seal lips and friction of the rotational bearings) and cooled areas (seawater contact area), this gives rise to a thermal flux through the components from the warmer areas to the cooler areas, and especially a thermal flux through the shaft bushing from the area of the lubricating oil and the area of the seal rings to the area in contact with the cooler seawater. Thus, the resulting temperature of the overall seal arrangement, and especially the temperature of the seal rings and the seal lips thereof, is predominantly determined by the thermal conductivity of the shaft bushing.
In addition to an adequate thermal conductivity, the shaft bushing must also satisfy other requirements, i.e. exhibit other characteristics. Namely, the shaft bushing carries out or ensures several functions, for example a corrosion protection of the propeller shaft relative to the corrosive seawater, and a defined counter running surface for the sliding contact with the seal rings having a prescribed and tested tribologic behavior, as well as the abovementioned conduction of heat away from the seal arrangement to the cooler seawater. These various different requirements or demands on the shaft bushing are substantially contrary to one another. Namely, a material of the shaft bushing that satisfies one of the above requirements, typically does not entirely satisfy the other requirements.
In order to increase the operating life of the seal rings and thus correspondingly increase the associated maintenance intervals, it has been recognized as desirable to reduce the operating temperature of the seal rings. An arrangement for achieving this has been proposed in the prior art, for example according to German Patent DE 41 35 709 C. That known arrangement uses heat pipes to conduct heat away from the area of the seal rings. A disadvantage of such a construction using heat pipes is the relatively high complexity, effort and expense for the production, construction, installation and maintenance of such a seal arrangement using relatively complex and technically demanding cooling elements. Furthermore, these heat pipe cooling elements have a relatively small cross-sectional area in the direction toward the heat sink, so that special measures or efforts must be taken in order to avoid a non-uniform distribution of the cooling agent and thus the cooling effect.